1. Field of the Invention
The present invention relates to a process for purifying propylene homopolymers or copolymers. More particularly, it relates to a process for purifying propylene homopolymers or copolymers which are produced by polymerization or copolymerization of propylene in liquid propylene (hereinafter referred to as "bulk polymerization" for brevity) in an industrially advantageous manner.
2. Discussion of the Prior Art
Bulk polymerization is well-known and has been widely applied in industry. In comparison with the polymer slurry produced by the conventional solvent (or suspension) polymerization (hereinafter referred to as "conventional solvent polymerization") in which polymerization is generally carried out in a liquid saturated hydrocarbon solvent having 5 or more and preferably 5 to 7 carbon atoms, the polymer slurry produced by the bulk polymerization using an unsaturated hydrocarbon monomer (mainly propylene) as a polymerization solvent, is far superior. The polymer is very easily separated from the solvent by merely treating the slurry under reduced pressure because the solvent has such a high vapor pressure and it is easily vaporized.
In general, however, the polymers obtained by bulk polymerization have some disadvantages. First, they generally contain a large amount of polymer soluble in boiling n-heptane. Second, they generally contain a large amount of catalyst residue. Accordingly, they have inferior physical properties as described below in more detail.
The polymer produced, for example, using a catalyst system of titanium trichloride and triethylaluminum contains boiling n-heptane-insoluble polymer in an amount of only 75 to 85%, and, in the case of a titanium trichloride and diethylaluminum chloride catalyst system, the amount of boiling n-heptane-insoluble polymer is 85 to 95%. Polymers containing boiling n-heptane-soluble polymer in large amounts have inferior physical properties, particularly stiffness and tackiness. Such polymers can be used as a moulding grade polymers, but they cannot be used as a film and fiber grade polymers requiring relatively high level physical properties.
Turning to the catalyst residues, reducing the catalyst residues is possible in itself in a bulk polymerization because the polymerization has a high polymerization rate in comparison to the conventional solvent polymerization and in addition it can be operated at a high catalyst efficiency. However, the fact is the polymer produced by this process is inferior in thermal stability and color and as a result it cannot be used as a high grade polymer.
Previously, the inventors attempted to raise the quality of such polymers as follows.
In the case of the polymer slurry produced by bulk polymerization, polymers soluble in the slurry, catalysts dissolved in the slurry among those remaining in the slurry and a part of catalysts deposited from the slurry can easily be separated from the slurry in the counter-current washing tower using liquid propylene as a washing solvent. As a result, a process was discovered for producing propylene homopolymers and copolymers in an industrially advantageous manner (Japanese Patent Application (OPI) Nos. 79589/1975, 102681/1975, 139886/1976 and 3679/1977 and U.S. Pat. No. 4,053,697.
With this process, however, there is a possibility of problems occurring when extremely higher levels of quality than commercially required at the present time are desired. In the above process, for example, additives such as a stabilizer and a lubricating agent are easily decomposed by the action of catalyst residues in the polymer, particularly chlorine components and therefore the use of large amounts of additives is necessary.
It is also well known that the epoxides used for stabilizing the catalyst residues deactivate the chlorine components, and in the case of the bulk polymerization of propylene, various purification processes using epoxides have been proposed. For example, Japanese Patent Publication Nos. 29946/1975 and 1274/1976 propose a process which comprises treating the polymer slurry by the direct addition of an expoxide and an alcohol thereto, followed by washing with a lower hydrocarbon. Every one of these processes, however, is not considered favorable, because the deactivation treatment is carried out in a system containing all of the titanium compound and an organo-aluminum compound used in the polymerization so that more treating agent is required. Further, when cheap, low-boiling propylene oxide is used as the epoxide, the separation of propylene from epoxide is so difficult that a rectifying tower of very high separation efficiency is required to purify the unreacted propylene for recycling to the polymerization step.
On the other hand, Japanese Patent Application (OPI) No. 139883/1975 discloses another process which comprises contacting a catalyst residue containing a polyolefin with a mixed vapor of an alkylene oxide and water and thereby purifying the polyolefin. This process is superior in removing the halogen components from the catalyst residues, but it is not considered favorable due to serious problems such as the corrosion of equipment owing to the hydrogen halide and moisture contained in the system.